As a duplex system in a mobile communication system using CDMA (Code Division Multiple Access), a TDD system is conventionally known. The TDD system uses the same frequency band for transmission and reception, called a ping-pong system, and is a system where time slots with the same radio frequency are used to communicate alternately over the uplink and downlink.
FIG. 1 illustrates an example of a configuration of a communication frame in the TDD system. The communication frame illustrated in FIG. 1 is divided into a plurality of times slots. In this communication frame, downlink time slots (downlink slots 1 to n) are configured in the first half of the frame, while uplink time slots (uplink slots 1 to n) are configured in the latter half of the frame. FIG. 1 illustrates an example where the uplink slots 1 to n are assigned corresponding uplink bursts 1 to n, while the downlink slots 1 to n are assigned corresponding downlink bursts 1 to n.
When a base station performs radio communications with terminal station apparatuses using thus configured communication frames, each terminal station apparatus is assigned an uplink slot and downlink slot contained in the communication frame. Based on a timing signal shown in FIG. 1, the base station apparatus switches between transmission processing and reception processing. A terminal station apparatus performs transmission processing and reception processing at timings of an uplink slot and downlink slot assigned to the terminal station apparatus, respectively.
Structures of an uplink burst and downlink burst will be described below with reference to FIGS. 2 and 3. FIG. 2 is a diagram illustrating a structure of a downlink burst, and FIG. 3 is a diagram illustrating a structure of an uplink burst. At a beginning of the downlink burst is disposed a propagation path estimation preamble that is a known signal and used in propagation path estimation. The propagation path estimation preamble is followed by data (downlink data #1 and #2) to a terminal station.
Downlink bursts are transmitted at respective predetermined timings from a base station apparatus, and therefore, never collide with one another. Accordingly, the downlink burst is not provided with a guard time.
Meanwhile, as shown in FIG. 3, the uplink burst is provided at its beginning with a guard time to prevent the collision with an adjacent burst, and the guard time is followed by a sync/AGC preamble used in detection of synchronization and AGC (Automatic Gain Control). The sync/AGC preamble is followed by a propagation path estimation preamble that is a known signal and used in propagation path estimation. The propagation path estimation preamble is followed by data (uplink data #3 and #4) to the base station.
In the communication frame illustrated in FIG. 1, configuring downlink bursts collectively shifts a reception timing of an uplink burst from a predetermined reception timing, and thereby prevents the collision with the downlink burst. In this way, since the need is eliminated of providing a downlink burst with a guard time, the rate of the guard time to the communication frame is decreased to improve the transmission efficiency.
In the mobile communication field, since the quality of received signals significantly deteriorates due to fading, the diversity processing is used to reduce the deterioration in the quality of received signals due to fading. The diversity processing is a technique of preventing drops in power of received signals on a receiver side, but in order for a communication terminal apparatus such as a mobile station to implement the diversity, there are limitations in processing capability, miniaturization, etc. Therefore, transmission diversity has been studied in order for a transmitter side to implement the diversity which is originally to be implemented on a receiver side.
The transmission diversity will be described which is performed between a base station and terminal station when the base station communicates with the terminal station using communication frames shown in FIG. 1. It is assumed that each of the base station and terminal station has an IFFT circuit, and performs OFDM communications over both the uplink and downlink.
The base station receives uplink bursts using a plurality of antenna elements, and detects received levels for each antenna element. The detection of received levels is performed for each subcarrier of a received signal. Based on a result of detection of received levels, the base station assigns a subcarrier composing a downlink burst to an antenna element with the highest received level with respect to the subcarrier.
Such transmission diversity has a premise that in the TDD system, propagation path characteristics are approximately the same over the downlink and the uplink. Based on this premise, in the transmission diversity, a downlink signal is transmitted from a branch with the highest uplink received power, whereby it is aimed to maximize the downlink received power in a terminal station.
However, in the frame configuration where downlink slots and uplink slots are respectively collected and arranged, since an uplink slot is spaced apart from a downlink slot (in other words, a large time interval exists between reception of an uplink slot and transmission of a downlink slot), the channel condition at the time of receiving the uplink slot is greatly different from the channel condition at the time of receiving the downlink slot. Accordingly, the premise that propagation path characteristics are approximately the same over the downlink and the uplink crumbles, and there arises a problem that the capability of diversity of improving the received quality deteriorates.